The present invention relates to power supply apparatus and methods of operation thereof, and more particular, to rectifier circuits and methods.
Rectifier circuits are commonly used in power supply apparatus, such as uninterrupted (or "uninterruptible") power supplies (UPSs), motor drives, and the like, to produce a DC voltage from an AC input voltage provided from an AC power source, such as a utility line. For example, in online UPS applications, a passive rectifier may be used to generate a DC voltage from a utility AC voltage, with the DC voltage being regulated and transformed back into an AC output voltage to provide a clean, regulated source of AC power for devices such as computers or telecommunications equipment.
One problem often associated with the use of passive rectifier circuits is the generation of undesirable levels of current harmonics at the AC input of the rectifier, i.e., currents having frequencies that are multiples of the fundamental frequency of the AC input voltage. For example, a typical passive three-phase rectifier circuit may generate high levels of fifth, seventh and higher order odd harmonics. The generation of such harmonics has become the subject of increasing concern, as these harmonic currents can result in the need for increased current carrying capability in the AC system.
Techniques have been proposed for reducing the magnitude of such harmonic currents. These techniques include the use of passive or active filters that are coupled to the input of the rectifier and which suppress the harmonic currents generated at the rectifier input. The generation of harmonics may also be reduced by using active rectification circuits, for example, switched rectifier circuits that can be controlled to reduce the magnitude of harmonic currents at the rectifier input.
Although these techniques can be successful in reducing harmonics, they can have various disadvantages. Passive filters typically are frequency selective, i.e., only suppress a selected harmonic. Consequently, multiple passive filter circuit may be needed to suppress multiple harmonics. Active filtering and rectification schemes may require complex switching networks and associated control electronics.